One particular type of self-service terminal is an automated teller machine (ATM). Traditional ATMs allow customers to withdraw banknotes from their accounts. Some advanced ATMs also allow customers to deposit banknotes, which are then recycled by the ATM and dispensed to a subsequent customer. These ATMs are sometimes referred to as currency recyclers. Currency recyclers are different to currency deposit ATMs, which merely store deposited banknotes for return to the owner of the ATM, in that a currency recycler dispenses banknotes that have been deposited by customers at that ATM.
When currency recyclers are used, a banknote may be dispensed and deposited on many occasions without ever being presented to a retail bank. Since banknotes degrade over time, currency issuing banks (typically central reserve banks) would like to remove from circulation those banknotes that are in poor condition. One aspect of poor condition is the soiling level of a banknote. It is therefore desirable for recycling ATMs to be able to ascertain the soiling level of a banknote and to remove it from circulation if it is below a certain minimum standard.
Ascertaining the soiling level of a banknote is not straightforward because the change to a banknote is not uniform for the most common types of banknote. For example, as paper banknotes (which are typically made from a cotton/paper substrate) become soiled, the dark areas of the banknote (typically intaglio printed areas) become brighter, whereas, the bright areas of the banknote (typically unprinted or watermark areas) become darker. This occurs because: (i) the fibres in the substrate (paper/cotton) break down, (ii) the ink wears off, and (iii) dirt is caught between exposed fibres. For banknotes made from a polymer substrate (polymer banknotes), the whole banknote tends to get brighter because ink wears off but there are no fibres to collect dirt.